Thông tin thiết kế mạch P11

Chia sẻ: Tien Van Van | Ngày: | Loại File: PDF | Số trang:42

lượt xem

Thông tin thiết kế mạch P11

Mô tả tài liệu
  Download Vui lòng tải xuống để xem tài liệu đầy đủ

PERSONAL WIRELESS COMMUNICATION SYSTEMS The idea that a person could carry around with him a telephone booth of his own is a very attractive one. However, the technology required to make the telephone booth small and light enough for this to be possible, and furthermore convenient to carry, has been available only in the last 30 years. Mobile radio has been available in North America since the 1930s but they were exclusive in the hands of the police.

Chủ đề:

Nội dung Text: Thông tin thiết kế mạch P11

  1. Telecommunication Circuit Design, Second Edition. Patrick D. van der Puije Copyright # 2002 John Wiley & Sons, Inc. ISBNs: 0-471-41542-1 (Hardback); 0-471-22153-8 (Electronic) 11 PERSONAL WIRELESS COMMUNICATION SYSTEMS 11.1 INTRODUCTION The idea that a person could carry around with him a telephone booth of his own is a very attractive one. However, the technology required to make the telephone booth small and light enough for this to be possible, and furthermore convenient to carry, has been available only in the last 30 years. Mobile radio has been available in North America since the 1930s but they were exclusive in the hands of the police. Later taxicab operators installed radios in their vehicles. These mobile units were large, heavy and electrical power hungry. They used amplitude modulation which is notorious for poor performance in the presence of electrical noise and there was more than enough noise generated by the ignition systems of the vehicles in which they were installed. Moreover, they were operational in either the transmit or the receive mode at any given time; they were of the ‘‘push-to-talk’’ type. The invention of the transistor and its progression to integrated circuits made it possible to reduce the weight and size of circuits and, at the same time, increase their capability and flexibility. These advances were accompanied by an enormous reduction in the amount of power required to operate transistor circuits. The stage was set for the introduction of the ‘‘personal telephone booth’’. There are many applications in which radio plays a vital part. These go from ‘‘remote keys’’ for the automobile, garage door openers, pagers, walkie-talkies, cordless telephones to cellular telephones with access to the Internet. In this chapter, we limit ourselves to a discussion of paging systems, cordless and cellular telephones. The paging system was designed to send information from a base station to a mobile terminal. The mobile terminal has no capability to transmit information in the opposite direction. A serviceman for home heating furnaces, for example, only needs to know the address of his next assignment; in general, he does not have to contact his base office. A communication system in which information travels in only one direction is described as simplex and the pager is an example. 325
  2. 326 PERSONAL WIRELESS COMMUNICATION SYSTEMS Radio systems with a push-to-talk button use the same channel in both the forward and reverse directions. It is therefore necessary for each person to indicate when they have finished talking with the familiar word ‘‘roger’’. They are described as half- duplex. A full-duplex system uses two channels simultaneously, the first for transmission and the second for reception. The cellular telephone is an example of a full-duplex system. 11.2 MODULATION AND DEMODULATION REVISITED In Section 9.2.1 we discussed the generation of a single-sideband-suppressed carrier (SSB-SC) signal using a balanced modulator and a bandpass filter. Fig. 11.1(a) shows the circuit configuration of the SSB-SC as well as the frequency spectrum of the input and output. The output shows that the baseband signal has experienced an upward frequency shift equal to the carrier frequency, oc and an ‘‘inverted’’ version of it appears at a lower frequency and the two are symmetrically spaced about the position of the carrier. A bandpass filter is used to select the upper sideband. Clearly, the upper and lower sidebands contain the same information and only one of them should be required for the recovery of the original signal. Figure 11.1(b) shows a circuit in which upper sideband is multiplied (balance modulated) with the carrier signal, oc . The corresponding spectrum shows that the Figure 11.1. (a) The structure of the modulator and the spectrum of the corresponding frequency shift. (b) The structure of the demodulator and the spectrum of the corresponding frequency shift.
  3. 11.3 ACCESS TECHNIQUES 327 output has two ‘‘sidebands’’. The first is at a frequency 2oc and the second occupies the position of the original baseband signal in the spectrum. The original signal is recovered by using a lowpass filter. Equations (9.2.1 to 9.2.5) are the relevant equations. This modulation and demodulation technique can be used with amplitude, frequency and phase or angle modulation schemes. When demodulation is carried out using the carrier signal in a balanced modulator as shown in Figure 11.1(b), it is referred to as coherent demodulation or coherent detection. The use of an envelope detector to demodulate an AM signal is known as non-coherent demodulation. 11.3 ACCESS TECHNIQUES 11.3.1 Multiplex and Demultiplex Revisited When modulation is used to accommodate a number of signals on a single channel we refer to it as multiplexing. Figure 11.2 shows five baseband signals, each of which occupies the frequency band 300 Hz to 3 kHz. By choosing suitable carrier frequencies for each one, they may be transmitted over the same cable or the airwaves by radio and subsequently demodulated with no interference between them. When different carrier frequencies are used to multiplex the baseband signals, it is referred to as frequency-division multiplex (FDM). Other methods of multiplexing are described below. The success of personal wireless communication systems is in part due to the development of techniques which allowed a large number of signals to share a limited spectrum. One of the boundaries of the spectrum available for personal wireless communication is dictated by the size of the antenna for the radio interface. Efficient transmission of radio signal at low frequency requires antennas several thousand meters tall. Clearly, this is not possible as portability of the device is essential. The boundary on the other end of the spectrum is set by the character of high-frequency transmission which increasingly takes on the properties of visible light which requires line-of-sight. Clearly, the modern environment (cities) in which most of the potential subscribers live and work make line-of-sight communication Figure 11.2. Five baseband signals occupying the same bandwidth can be separated by using frequency-division multiplex.
  4. 328 PERSONAL WIRELESS COMMUNICATION SYSTEMS devices inadmissible. Between these two boundaries we have other systems in competition for the spectrum, such as air and sea navigation, satellite communica- tion, radio and television broadcasting. It so happens that national governments have arrogated to themselves the power to assign portions of the spectrum for specific purposes within their territories and to negotiate international treaties which govern their use. This, in short, brings us to the assigned frequency bands of 824–849 MHz and 869–894 MHz for personal wireless communication. For the large number of anticipated subscribers to be accommo- dated in such a restricted bandwidth it is necessary to develop techniques which reduce the possibility of interference with each other. One major factor working in our favor is that, provided we keep the radiated power below a given level, and we are separated sufficiently by distance, we can reuse the spectrum over and over again. We shall now discuss the techniques which enable us to share the spectrum available. 11.3.2 Frequency-Division Multiple Access (FDMA) Frequency-division multiple access is a fancy name for what is commonly done with AM and FM radio broadcasting and TV stations; they are assigned different carrier frequencies with suitable separation between them to ensure minimal interference with each other. They are required by law to keep their carrier frequencies constant. They also have a limited bandwidth and radiated power. The division of the spectrum according to frequency was discussed in Section 9.2 under the heading ‘‘Frequency- Division Multiplex’’ (FDM). Figure 11.3 shows a representation of the channels spaced by their assigned carrier frequencies and separated by limited bandwidth and appropriate guard bands. 11.3.3 Time-Division Multiple Access (TDMA) In FDMA, a frequency band is dedicated to a particular channel for as long as it is required. In TDMA, several channels share the same bandwidth but each channel has the use of that bandwidth for a fraction of the time. TDMA was discussed in Section 9.3 under the other name used to describe this technique: ‘‘Time-Division Figure 11.3. In frequency-division multiple access (FDMA), channels are spaced by their assigned carrier frequencies and separated by limitation on bandwidth and appropriate guard bands.
  5. 11.3 ACCESS TECHNIQUES 329 Multiplex’’ (TDM). The basis of this technique is the ability to reconstruct a signal from samples taken from it. Figure 11.4 shows how each channel is structured in time to form frames and the sequences of the content of each channel. In TDMA, it is necessary to synchronize the transmitter to the receiver so that bits from one channel do not end up in another channel, hence the synchronizing bits. 11.3.4 Spread Spectrum Techniques In spread spectrum communication systems the radio-frequency carrier is changed very rapidly in a pseudo-random fashion over a bandwidth which is much wider than the minimum required to transmit the signal. Potentially it should cause interference with other users of the airwaves but, in fact, because the carrier operates for such a short time at any given frequency, its effect is almost imperceptible. The average perceived power on any given channel is very low and it therefore behaves like a low-power noise source spread across the bandwidth it uses. Many communication channels can operate in this fashion without interfering with each other. Spread spectrum technology has been of particular interest to the military because it is almost impossible to predict the next frequency of the transmission; they like to stay away from eavesdroppers and to avoid the jamming of their communication systems by the enemy. The real challenge in spread spectrum communication is to keep the receiver synchronized to the transmitter. We shall return to the problem of synchronization later. There are two major types of spread spectrum techniques. They are frequency hopped and direct sequence spread spectrum technologies. Frequency Hopped Multiple Access (FHMA). In FHMA trans- mission, the information is first digitized and then broken up into short passages. Each passage is transmitted on a different carrier frequency determined by a pseudo- random number generator. Because the modulation used is either narrow band FM or frequency-shift keying, at any instant, a frequency hopped signal occupies a single narrow channel. However, because the carrier frequency hops around, it makes use of a much wider bandwidth. Figure 11.5 shows a representation of a system that uses FHMA. Clearly, in an FHMA the receiver has to have prior access to the sequence of the carrier frequencies transmitted as well as the timing to be able to follow the hops (synchronize). It is quite likely that two or more transmitters will at some time try to use the same frequency. Code Division Multiple Access (CDMA). In CDMA transmission, the information is first digitized and then multiplied by a binary pseudo-random sequence of bits (called chips) with a bit rate much higher than that of the digitized information [1]. Figure 11.6 shows the binary message signal, bits of a pseudo- random code, and the spread spectrum (coded) signal. Note that, because the bit rate of the pseudo-random sequence is much higher than that of the message signal, it requires a much larger bandwidth for its transmission. The spread signal is used to modulate a carrier (usually FM or PM)
  6. 330 Figure 11.4. In time-division multiple access (TDMA), each channel is structured in time to form frames and the sequence of the content of each channel.
  7. 11.4 DIGITAL CARRIER SYSTEMS 331 Figure 11.5. A representation of a frequency hopped multiple access (FHMA) system. Although no instances of two or more transmission on the same frequency and at the same time are shown, there is a clear possibility that this can happen. Note that for simplicity, all channels have equal bandwidth and occupy that bandwidth for the same length of time. Neither of these conditions apply in practice. and then transmitted. At the receiving end the spread signal is demodulated then decoded using a locally generated pseudo-random bit sequence in a process called correlation. Because the number of chips representing a message bit (1 or 0) is large, the correlation does not have to be perfect; it has to correctly recognize the majority of the chips as representing that message bit (1 or 0). In a system which is subject to multipath fading, this is an advantage. It is clear that the receiver has to have prior knowledge of the pseudo-random code to be able to decode the message. To other receivers not using the identical code, the message appears to be just noise. The attraction of this technique is that it can be used to accommodate a large number of subscribers with different codes and they will not even know that they are sharing the same bandwidth. A by-product of CDMA is improved security of the message. One disadvantage of CDMA is that the power of individual transmitters has to be controlled very carefully. A strong signal from one of the transmitters within the wideband can overwhelm the sensitive front-end of the system and prevent the reception of other signals. The transmit power control system for all the mobiles adds complexity and costs. 11.4 DIGITAL CARRIER SYSTEMS So far, we have discussed carrier systems in which the message signal is in analog form. Increasingly, electronic systems are using a digital format. For example, it has
  8. 332 Figure 11.6. The binary data and its equivalent bipolar form, the pseudo-random sequence (chips) and the resulting spread spectrum data.
  9. 11.4 DIGITAL CARRIER SYSTEMS 333 taken the music recording industry less than 15 years to replace the analog vinyl record with the digital compact disc. There are technical as well as economical advantages to be gained from this move. Moreover, the advent of integrated circuit technology with its ability to fabricate extremely large numbers of circuits on minuscule pieces of semiconductor has made the move to digital systems seem inevitable. To transmit a baseband (message) signal over a radio channel, it is necessary to change some property of the radio-frequency signal using the baseband signal. We can change its amplitude, its frequency, or its phase angle. In Chapter 2 we discussed the modulation of a radio-frequency signal by a message signal in which the amplitude of the RF signal varied according to the amplitude of the message signal (amplitude modulation; AM). In Chapter 4 we discussed how to change the frequency of the RF about a fixed value using the message signal (frequency modulation; FM). It is now time to discuss the modulation scheme in which we vary the phase of the RF signal according to the message signal (phase modulation; PM). It must be pointed out that frequency and phase modulation are, in fact, the same. The only difference is that in PM the phase of the modulated waveform is proportional to the amplitude of the modulating waveform, while in FM it is proportional to the integral. Both schemes are sometimes referred to as angle modulation. Phase modulation, when the message signal is a continuous (analog or tone) function, does not appear to have any practical applications. When the message signal is digital, it has distinct advantages such as improved immunity to noise. 11.4.1 Binary Phase Shift Keying (BPSK) When the modulating (message) signal is in binary form we refer to it as keying. This is a left-over from the days when telegraph operators opened and closed a circuit (presumably, using a ‘‘Morse key’’) to generate Morse code. Figure 11.7 shows a comparison of the waveforms of the three modulating schemes. It should be noted that, for clarity, the RF has been chosen to be only four times the data rate. In practice, the RF is much higher than the data rate. If we represent the digit 1 by the binary pulse pðtÞ ¼ 1, the digit 0 by pðtÞ ¼ À1 and the carrier by cos oc t, then after modulation we have sðtÞ ¼ pðtÞ cos oc t ð11:1Þ for the digit 1 and sðtÞ ¼ ÀpðtÞ cos oc t ¼ pðtÞ cosðoc t þ pÞ ð11:2Þ for the digit 0. Demodulation of a BPSK signal requires a balanced mixer and an exact replica of the carrier.
  10. 334 PERSONAL WIRELESS COMMUNICATION SYSTEMS Figure 11.7. (a) Binary data, (b) its bipolar equivalent, (c) the amplitude-modulated waveform, (d) the frequency-modulated waveform, and (e) the phase-modulated waveform. 11.4.2 Quadrature Phase Shift Keying (QPSK) In quadrature phase shift keying, the message signal is separated into in-phase (I) and quadrature-phase (Q) components and are then modulated separately by two carriers of the same frequency but with a phase difference of 90 . QPSK is used because twice the information can be carried in the same bandwidth as when BPSK is applied [2]. Figure 11.8 shows the structure of the QPSK modulator. It has been assumed that the pulses used for modulating the carrier are rectangular. In fact, rectangular pulses are quite undesirable since, in a limited bandwidth channel, they tend to smear into the time intervals of other pulses [3]. The pulse shaping filter is used at the baseband or at the IF stage to limit adjacent channel interference. Figure 11.9 shows the waveforms of the original data, the I and Q components, the I cos ot and Q sin ot as well as the QPSK signal. Note that the QPSK signal is a combination of the waveforms of the I cos ot and Q sin ot components. The demodulation of the QPSK signal is done coherently as shown in Figure 11.10. After down-conversion the received signal is split into two parts and each part is demodulated using a carrier signal derived from the received signal by a carrier
  11. Figure 11.8. Block diagram of the QPSK modulator. 335
  12. 336 PERSONAL WIRELESS COMMUNICATION SYSTEMS Figure 11.9. The waveforms of (a) the data, (b) the non-return-to-zero in-phase (I) component, (c) the non-return-to-zero quadrature (Q) component (note that the waveform shown in (a) is not coincident in time with those shown in (b) and (c) ), (d) I cos ot, (e) Q sin ot, (f ) the QPSK signal, and (g) the QPSK signal with the phase shifted by þp=4.
  13. Figure 11.10. Block diagram of the QPSK demodulator. 337
  14. 338 PERSONAL WIRELESS COMMUNICATION SYSTEMS recovery circuit. The low-pass filters remove the undesirable products of the multiplication process. Two circuits make decisions on whether the bit that was sent was a 1 or a 0. The I and Q components are passed to a multiplexer which reconstitutes the original binary signal. 11.5 THE PAGING SYSTEM There are a number of occupations in which the professional has to move around from one job to the next and essentially is almost never available at a wireline telephone. The paging system is designed to receive and store information until the professional is ready to read it. They are most commonly used by home appliance servicemen, office equipment servicemen, doctors, photographers, and in the last few years they have become very popular with teenagers. Different paging systems have varying capabilities. The message received and stored may be as simple as ‘‘Call the paging center to pick up your message’’ or it may give the number of the caller or an alphanumeric message, or in some of the more sophisticated systems the caller can leave a voice message. The important difference between paging and other systems of communication is that in paging, there is no need for an immediate response. 11.5.1 The POCSAG Paging System In this section, we discuss the design and operation of one of the simpler paging systems currently in use. This is the Post Office Code Standardization Advisory Group (POCSAG) system. This system was introduced in the early 1980s as a standard for the manufacture of pagers for the British Post Office. It can handle up to 2 million addresses per carrier and supports tone only (alert only), numeric, and alphanumeric pagers. There are three speeds at which the POCSAG system transmits its messages; they are 512, 1200, and 2400 bps. These data rates would normally be considered to be slow but this is deliberate because, combined with high transmitter power (hundreds of watts to a few kilowatts), it improves reliability. The message is ‘‘broadcast’’ over the entire area of operation and it is supposed to reach the recipients whether they are in a building, on a highway, or in an airplane. Typical carrier frequency of operation of the transmitters is around 150 MHz. Each message is preceded by a CAP code which is a unique 7 or 8 digit code recognizable to only one paging receiver in the geographic area of operation. The Paging Transmitter. Figure 11.11 shows a block diagram of the transmit portion of the paging system. Most of the messages come in over the telephone system. The source of the message can be from a Touch-tone1 telephone whose keypad can be used to enter the information to be transmitted. It can be a voice message, in which case a human dispatcher in the paging center has to intervene and key in the appropriate message. The message can also come from a computer with the appropriate software and a modem. Whatever its source or form,
  15. 11.5 THE PAGING SYSTEM 339 Figure 11.11. Block diagram of the transmit portion of the paging system. the message goes into an A=D converter. The digital output is used to drive a frequency shift keying encoder in which the digit 0 is assigned a frequency of, say, 1200 Hz and the digit 1 is represented by a tone of frequency 2400 Hz. The message is placed in a queue with other messages. The appropriate CAP code is inserted ahead of each message frame. The dual tone signals may be sent over landlines or wireless systems to a large number of frequency modulated transmitters distributed over a geographic area. Component Circuit Design. The function of the ‘‘processor’’ is to condition the analog signals coming over the telephone line into the paging center for the A=D converter. The human dispatcher plays the same role. The design of the A=D converter is described in Section The frequency shift keying encoder is a form of modem. Modem circuits are described in Section 9.4.1. The frequency modulated (FM) radio transmitter was the subject of Chapter 4. The Paging Receiver. The block diagram of the basic paging receiver is shown in Figure 11.12 [4]. The paging receiver is typically a small device which can be worn on a waist belt. It is basically an FM receiver with a fixed carrier frequency. It has an internal antenna typical of portable radios. Each receiver has a unique CAP code programmed into it and when a message arrives with the appropriate CAP code, the message is saved in the memory and the controller triggers the alert generator which sends a signal to the alert transducer. All other messages are ignored. The alert may be in the form of a sub-audio vibration, a
  16. 340 PERSONAL WIRELESS COMMUNICATION SYSTEMS Figure 11.12. Block diagram of the paging receiver. chime, a beep, or a short excerpt of a well known tune. For a ‘‘tone only’’ paging receiver, the wearer is simply alerted and has to place a call to a messaging center to get the message. For numeric and alphanumeric receivers, the stored message can be retrieved by pushing the appropriate buttons on the front of the device. On receiving the appropriate commands from the keypad, the controller causes the output data control to send the stored information to the decoder which converts it into a form suitable for display on the liquid crystal display (LCD). The messages remain in the memory until they are cleared. Component Circuit Design. The frequency modulated radio recei- ver was the subject of Chapter 5. A frequency shift keying decoder was discussed in Section 9.4.1 (modems). Memory circuits, their control, coding, and decoding are discussed in Appendix E. Liquid Crystal Display. Certain chemical compounds, such as the cyanobiphenols, have the property that causes the rotation of polarized light passing through them. These compounds are normally transparent to visible light but when seen in a container they appear to be translucent. This is because the axes of the molecules are normally randomly oriented and hence they scatter light in random directions. When an electric field is applied to the compound, the axes of the molecules line up and, depending on the orientation of the incident polarized light, they allow the light to go through or stop it [5]. It is possible in some of the most commonly used liquid crystals to make the light ‘‘twist’’ or gradually change its orientation as it travels through the liquid. This phenomenon is known as twisted nematic and the degree of twist can be set during manufacture. This is the basis of the common liquid crystal display which is used in watches, pagers, and many other electronic consumer goods.
  17. 11.5 THE PAGING SYSTEM 341 Figure 11.13 shows two parallel transparent electrodes with backings of polariz- ing film, one vertically oriented and the other horizontally oriented. The space between the electrodes is filled with the liquid crystal compound. When the light enters the vertically polarized film, only the vertical component will pass through and enter the liquid crystal. As it travels from the right-hand electrode to the left-hand electrode its orientation is changed from the vertical to the horizontal. If the left-hand side polarizing film is oriented horizontally, the light will pass through it. When an electric field is applied across the electrodes the axes of the molecules are lined up such that they do not affect the orientation of the light. The vertically polarized light cannot go through the horizontally polarized left-hand side film. The contrast created by the presence or absence of the electric field is exploited in the application of the liquid crystal as a display transducer. In the display of alphanumeric characters, the most commonly used units are the seven-segment and the dot-matrix displays [6]. The dot-matrix display is the more versatile of the two but its decoding system is more complex. The decoding of information in a binary format for display by the relatively simple seven-segment display is not as simple as might be expected and an explanation of how it is designed will be an unnecessary diversion at this point. The seven-segment LCD and its driver, the MC5400=7400 series integrated circuit, are presented in Appendix F. 11.5.2 Other Paging Systems Since the early 1980s when the POCSAG system was introduced, a number of new systems have been developed. They have increased the speed of transmission, lowered the current drain on the battery, increased the number of addresses per carrier, improved reliability, and made the system more difficult to tamper with. Two of these systems are described very briefly below. (1) ERMES (European Radio Message System). This system was introduced in the early 1990s by the European Community. The data rate is fixed at 6250 bps and it is capable of operating over multiple radio-frequency channels. The pager can scan all the channels when the subscriber is away from his home base. (2) FLEXTM (Flexible wide-area paging protocol). This system was intro- duced in 1993 as a high performance multi-speed paging protocol (1600, 3200 and 6400 bps). FLEX can support over 5 Â 109 addresses and conserves the pager battery life by sending data in specified time slots only [7]. At the end of the 20th century it was estimated that there were 192 million pages in use world-wide [8].
  18. 342 Figure 11.13. An illustration of the operation of the liquid crystal as a display transducer. Reprinted with permission from W. C. O’Mara, Liquid Crystal Flat Panel Display, Van Nostrand Reinhold, New York, 1993.
  19. 11.6 THE ANALOG CORDLESS TELEPHONE 343 11.6 THE ANALOG CORDLESS TELEPHONE The cordless telephone was designed to liberate the telephone user from the tether that the handset cord is. Before cordless telephones appeared on the market, long handset cords were used to increase the distance between the handset and the base but the longer the cord got the more clumsy it became. The cordless telephone not only increased the distance from the base, it made the handset completely portable. 11.6.1 System Design Figure 11.14 shows the configuration of the cordless telephone. The base station is connected directly to the Public Switched Telephone Network (PSTN) and, from the point of view of the PSTN, it is just another telephone set. In fact, part of it is a transceiver which provides a two-way link to the handset. It can transmit signals to the handset and receive signals from the handset for onward transmission to the PSTN. The telephone part of the system is the same as any wireline telephone set (see Chapter 8) and the radio part of it uses frequency modulation (see Chapters 4 and 5) in the 900 MHz band. The base station transmitter operates at one frequency (say, 925.997 MHz) while the handset transmitter operates at another frequency (say, 902.052 MHz). This is an example of two simplex systems which form a frequency- division duplex (FDD). A device called a duplexer provides a coupling between the antenna and both the transmitter and the receiver. Other frequencies were used in the past and a new generation of cordless telephones, using digital technology, have been assigned spectra in the 2.4 GHz band in North America. The handset antenna is coupled to both the FM transmitter and receiver by the duplexer. Separate amplifiers condition the signal from the microphone and the signal going to the speaker appropriately. 11.6.2 Component Design The designs of all the components in Figure 11.14 were discussed earlier with the exception of the duplexer. The Radio-Frequency Duplexer. The role of the RF duplexer is to couple the strong signal from the transmitter to the antenna with minimal loss but prevent the transmitter signal from reaching the input of the receiver. In many applications, such as radar and the wired telephone system, a hybrid performs this function (see Sections and 8.4.3). The duplexers in both the base station and the handset have to ensure that the path from their transmitter to their receiver has the highest possible attenuation so that no significant local feedback is possible. At the same time they must ensure that the path from the transmitter to the antenna and that from the antenna to the receiver have minimum attenuation. There are two important factors to consider. The fact that the transmit and receive frequencies are separated by a fairly wide margin of approximately 25 MHz is a great help in the design. The fact that the signal power from the transmitter of the handset to its
  20. 344 Figure 11.14. Block diagram of the cordless telephone: (a) the base station, (b) the handset.
Đồng bộ tài khoản